Re: [PATCH 2/2] hw/core/loader: workaround read() size limit.

["Philippe Mathieu-Daudé" <philmd@redhat.com>]

On 11/11/21 16:36, Jamie Iles wrote:
> Hi Philippe,
> 
> On Thu, Nov 11, 2021 at 03:55:48PM +0100, Philippe Mathieu-Daudé wrote:
>> Hi Jamie,
>>
>> On 11/11/21 15:11, Jamie Iles wrote:
>>> On Linux, read() will only ever read a maximum of 0x7ffff000 bytes
>>> regardless of what is asked.  If the file is larger than 0x7ffff000
>>> bytes the read will need to be broken up into multiple chunks.
>>>
>>> Cc: Luc Michel <lmichel@kalray.eu>
>>> Signed-off-by: Jamie Iles <jamie@nuviainc.com>
>>> ---
>>>  hw/core/loader.c | 40 ++++++++++++++++++++++++++++++++++------
>>>  1 file changed, 34 insertions(+), 6 deletions(-)
>>>
>>> diff --git a/hw/core/loader.c b/hw/core/loader.c
>>> index 348bbf535bd9..16ca9b99cf0f 100644
>>> --- a/hw/core/loader.c
>>> +++ b/hw/core/loader.c
>>> @@ -80,6 +80,34 @@ int64_t get_image_size(const char *filename)
>>>      return size;
>>>  }
>>>  
>>> +static ssize_t read_large(int fd, void *dst, size_t len)
>>> +{
>>> +    /*
>>> +     * man 2 read says:
>>> +     *
>>> +     * On Linux, read() (and similar system calls) will transfer at most
>>> +     * 0x7ffff000 (2,147,479,552) bytes, returning the number of bytes
>>
>> Could you mention MAX_RW_COUNT from linux/fs.h?
>>
>>> +     * actually transferred.  (This is true on both 32-bit and 64-bit
>>> +     * systems.)
>>
>> Maybe "This is true for both ILP32 and LP64 data models used by Linux"?
>> (because that would not be the case for the ILP64 model).
>>
>> Otherwise s/systems/Linux variants/?
>>
>>> +     *
>>> +     * So read in chunks no larger than 0x7ffff000 bytes.
>>> +     */
>>> +    size_t max_chunk_size = 0x7ffff000;
>>
>> We can declare it static const.
> 
> Ack, can fix all of those up.
> 
>>> +    size_t offset = 0;
>>> +
>>> +    while (offset < len) {
>>> +        size_t chunk_len = MIN(max_chunk_size, len - offset);
>>> +        ssize_t br = read(fd, dst + offset, chunk_len);
>>> +
>>> +        if (br < 0) {
>>> +            return br;
>>> +        }
>>> +        offset += br;
>>> +    }
>>> +
>>> +    return (ssize_t)len;
>>> +}
>>
>> I see other read()/pread() calls:
>>
>> hw/9pfs/9p-local.c:472:            tsize = read(fd, (void *)buf, bufsz);
>> hw/vfio/common.c:269:    if (pread(vbasedev->fd, &buf, size,
>> region->fd_offset + addr) != size) {
>> ...
>>
>> Maybe the read_large() belongs to "sysemu/os-xxx.h"?
> 
> I think util/osdep.c would be a good fit for this.  To make sure we're 

Yes.

> on the same page though are you proposing converting all pread/read 
> calls to a qemu variant or auditing for ones that could potentially take 
> a larger size?

Yes, I took some time wondering beside loading blob in guest memory,
what would be the other issues you might encounter. I couldn't find
many cases. Eventually hw/vfio/. I haven't audit much, only noticed
hw/9pfs/9p-local.c and qga/commands-*.c (not sure if relevant), but
since we want to fix this, I'd rather try to fix it globally.